Purification and Characterization of Amylase-Binding Tryptic Fragments from Streptococcus gordonii G9B. Previous studies have demonstrated that the binding of amylase to the surface of Streptococcus gordonii G9B is trypsin sensitive. The goal of the present study was to purify and characterize amylase-binding tryptic fragments from Streptococcus gordonii GAB. Cells were cultured in FCC defined media, trypsinized and centrifuged, and the supernatants were dialyzed and lyophilized. The resulting material was chromatographed on a Sephadex G-75 column, and fractions were analyzed by spectrophotometry at A280. Fractions containing immunoreactive components, as determined by immunoblotting using an antiserum generated against an alkali-soluble amylasebinding component (Infect. Immun. 1992. Vol 60, p. 4726-4733), were confined to a single included peak, the constituents of which demonstrated an approximate molecular weight of 10 kDa. These fractions were pooled, dialyzed and lyophilized. N-terminal sequence analysis suggested the presence of multiple N-termini. This was confirmed by 2D-PAGE in which multiple fragments, all of which were approximately 10 kDa, displayed isoelectric points ranging from 4.0 to 4.4. These components were then separated by FPLC Mono-Q chromatography. Analysis of fractions revealed the presence of at least six immunoreactive components. Chemical analysis revealed differences in amino acid and amino sugar compositions among the components. Negligible amounts of muramic acid and neutral sugars were found. All of these immunoreactive components were found to inhibit [125Ila-amylase binding to S. gordonii G9B. These results indicate that an amylase receptor has been isolated that is likely protein in nature.